The present invention generally relates to tie rods and, more particularly, to composite tie rods.
Tie rods have been used extensively in many products such as turbofan engines, air cycle machines, and turbomachinery rotors. Tie rods have been used to maintain component axial placements along a rotor, and to keep all components tight for smooth running. Prior art tie rods are single piece metal rods. Composite tie rods having increased structural dampening, reduced weight, increased stiffness and increased axial preload- and torque-carrying capacities are a bundle of fiber filaments collectively performing the task of a solid metal tie rod.
A carbon fiber drive shaft has been disclosed in U.S. Pat. No. 4,089,190. The tubular shafts are formed using conventional sheath winding techniques. Carbon fibers are impregnated with a resin binder and wound on a mandrel to produce a lightweight shaft having noise and vibration dampening characteristics. Unfortunately, the integrity of these shafts may not be sufficient for some high load applications.
Another shaft has been disclosed in U.S. Pat. No. 4,838,833. The described composite shaft uses reinforcing ribs to improve resistance to torsional buckling. Although the ribs may reduce torque induced shaft deformation, they may not be adequate for high axial load applications such as tie rods in tension.
A shaft having a metal tie-bar (i.e., tie rod) is described in U.S. Pat. No. 6,398,658. The shaft is formed from dissimilar materials, such as a ceramic and a metal, in coaxial alignment. The material least subject to radial expansion, for example the ceramic, is provided with an integral coaxial male member. The material more subject to radial expansion, for example the metal, is provided with an integral female skirt member and annular upstands. The female skirt member is fitted over the male member and the annular upstands are used to maintain the coaxial alignment of the ceramic and metal parts. Annular upstands at the peripheral edge are held in place by the compressive force of a tie-bar. Unfortunately, the described shaft is a rotating shaft for automotive use and has a tight radial clearance requirement along the shaft length. Although some properties of the '658 shaft may benefit from the use of a ceramic member, the brittle ceramic may not be suitable for other applications.
Rotors comprising a metal tie rod have been described. A metal rotor assembly 33 is shown in FIG. 1. The metal rotor assembly 33 has been used in high speed rotating apparatus. The purpose of the metal tie rod 30 is to hold every component 34 along the complete rotor assembly 33 in compression during operation. The metal tie rod 30 is provided with an end nut 31 and a bolt head 32. To achieve the necessary compression, the metal tie rod 30 (with all the components 34 mounted) is stretched to the required length by a hydraulic extensioner or by tightening the end nut 31. The induced compression (transmits from the end nut 31 to the components 34 along the rotor assembly 33) is provided by the metal tie rod 30 in tension. Unfortunately, the metal tie rod 30, which usually comprises steel, has a high thermal coefficient. The metal tie rod 30 experiences thermal growth as operating temperatures increase, which in turn reduces the compressive force along the shaft, rendering it inadequate for some high performance applications, unless a very high tensile preload were used. Further, the additional machining of complex changing diameters along steel tie rods' lengths increase operating costs, and weights.
As can be seen, there is a need for improved tie rods. A tie rod having a low thermal coefficient, reduced weight, and increased strength is needed.